Mechanical durability is a key feature of RFID transponders used in passports which experience significant mechanical stresses during their life cycle. The life cycle of passports is usually ten years and they must remain functional during this period. This constraint does not only apply to their physical integrity but also to the electrical function of the enclosed RFID transponder.
Repeated bending of the passport is one of the major threats to the enclosed RFID transponder. Indeed, the passport can be bent when carried in a backpocket, in pockets of a jacket or coat and repeated bending results in buckling of the RFID antenna which then causes a break of the antenna or such an alteration of the antenna's electrical properties which then results in a failure of the transponder.
Broken antennas occur in particular in passports which have a transponder enclosed in the passport cover. Indeed, the cover has a tendency to buckle when bent, which forces the enclosed antenna to buckle as well. Further bending then concentrates the bending stress on the area of buckling which rapidly causes a failure of the antenna and therefore of the entire transponder.
There are different approaches to minimize the buckling of a passport cover. One approach is to support the cover with a rigid and stiff layer. This thus prevents the passport from being bent. However, most countries do not accept a “hard” passport and prefer the haptics of a flexible passport booklet.
Another approach is to use textile based cover material. Textile materials can be bent without buckling. However, one disadvantage of such textile based cover material is their high costs. Another disadvantage is that textile cover materials are more difficult to emboss with a security embossing than paper based cover materials.
Today, most commonly used solutions are paper based cover materials because of their low cost and their good properties with respect to security technologies like embossing, security fibers, blanchetes etc. The downside of these materials is that they easily buckle when bent causing antenna failures and hence only provide a low resistance against passport bending.
Document DE19710656 discloses a smart card includes electronic component(s) and a base sheet. The invention there was the task of creating a smart card which is easy to manufacture and which avoids the tendency to gaping during a prolonged use. To this effect, a core sheet is joined to the base sheet and the core sheet has one or more regions stamped out to accommodate the component(s), including e.g. an antenna coil. The component is embedded with a filler in the stamped-out region the filler being initially in fluid form.
Document U.S. Pat. No. 6,630,370 relates to a process for manufacturing an IC card. More particularly, the present invention relates to a process for manufacturing an IC card having a plane coil with bent portions in which a conductor line is wound a plurality of times on substantially the same plane, and terminals of the plane coil and electrode terminals of a semiconductor element are electrically connected to each other. The idea here is to provide a method of manufacturing an IC card and an IC card avoiding a short circuit is caused by deformation of the conductor lines generated by an external force given to the plane coil in the traverse direction. The card comprises tape members which are bonded to a plurality of positions of the plane coil. The tape members are composed in such a manner that an adhesive layer is formed on one side of the tape member. In a portion to which this tape member is bonded, adhesive which has entered a space between the wound conductors is solidified, so that the conductors can be fixed while a predetermined interval can be provided between them. Therefore, even if an external force is given to the plane coil in the traverse direction, each conductor is not deformed. Accordingly, the occurrence of a short circuit caused by contact between the conductors can be prevented.
Document US 2007/0278315 relates to a flat transponder having an electronic circuit which is arranged in a layer or in a layer composite and which contains at least one chip and conductor tracks or conductor wires. The invention here is based on the object of specifying a transponder which is flexible, has the most uniform thickness possible and can be bonded to further elements by means of water-based adhesives, its electronic components being protected against damage by bending and against being detected by feel. One or both covering layers and, if appropriate, the partial layers of plastic arranged adjacently above it or under it are notched with incisions, so that the intrinsically stiff laminate with the circuit located in the interior of the laminate bends over at the notched points under bending loading, even with relatively low forces. The notches may be produced by drawn or punched incisions using special knives, by grinding, by sawing in, by laser cutting, etc.
A first notch pattern may be notched in with the lamination, and a second notch pattern can be notched in by means of additional notching following the lamination.
The notch depth and the notch cut density can be applied differently for regions of the laminate surface. In order not to be able to detect the module or chip in the laminate by feel and to prevent the conductor wires or conductor tracks from bending over at the module or chip connection, it is preferred not to flexibilize the regions surrounding the module or chip or to flexibilize them only slightly, in order additionally to make detecting the module or chip by feel more difficult.
Document US 2009/0291271 relates to a functional laminate for smart cards, ID cards, credit cards and the like which is able to absorb mechanical stress caused by shrinkage of the cover layer in the laminating process. To this effect, it is made from at least two co-laminated layers, at least one of which is a patchwork layer consisting of zones of multiple types distinct from each other such that mechanical stress may be absorbed by the zones when the functional laminate is subjected to heat thus keeping the functional laminate from breaking or delaminating. Such a functional laminate may be used in smart cards leading to an increased service life.
Document WO 2010/094790 relates to a functional laminate which is able to absorb mechanical stress caused by shrinkage of materials used in the laminating production process. According to an embodiment of the invention of this document, a method for manufacturing a functional laminate comprises at least the following steps:    providing an inlay formed of at least two layers which have been laminated together with a functional element, such that the functional element is at least partially embedded in the inlay;    forming at least one recess on the inlay in an area adjacent to the functional component;    laminating the inlay with at least one additional layer in such a manner that the width of said recess or recesses is (are) considerably reduced or said recess or recesses is (are) substantially closed at least by surrounding material after lamination by virtue of shrinking of the functional laminate.
In a functional laminate manufactured by this method, mechanical tensions due to heat induced shrinking of the substrate sheet or the at least one additional layer are reduced or removed by the recesses that are substantially closed or whose width is considerably reduced after lamination. Hence, the functional component and its contact areas, if applicable, are not subjected to these tensions, neither during the lamination procedure nor after. A risk of failure of the functional components due to cracks or warping is thus tremendously reduced. The recesses are formed at the latest before a final lamination step of the functional laminate. The characteristic of the final functional laminate is not significantly affected by the recesses since they are at least almost shut after the lamination step.
Safety documents and other plastic cards incorporating such a functional laminate may thus exhibit an increased service life and an improved optical quality by reduced surface unevenness.
Document DE 10 2004 027 978 discloses a semiconductor chip module which is located with an antenna between cover sheets. The chip is connected by tracks to the antenna terminals. Antenna and module are embedded in adhesive between the cover sheets (5) in one region.